1. Field of the Invention
The present invention relates to nickel-base alloys for isothermal forging dies operable at high temperatures in the atmosphere. More particularly, it relates to nickel-base superalloys for use in dies for isothermal forging at high temperatures of 1000.degree.-1150.degree. C. in the atmosphere.
2. Description of the Prior Art
Conventional dies for isothermal forging at temperatures of 1000.degree. C. or higher are made of molybdenum-base alloys such as TZM (0.5% Ti, 0.08% Zr and Bal. Mo). These alloys, however, have poor oxidation resistance, so forging operations must be conducted in vacuum or in an inert atmosphere. This makes isothermal forging facilities rather complicated and costly, leaving the forging operations inefficient and costly.
Nickel-base superalloys such as MAR-M200 (60% Ni, 9.0% Cr, 10.0% Co, 12.0% W, 5.0% Al, 2.0% Ti, 1.0% Cb, 0.15% C, 0.015% B and 0.05% Zr by weight) are widely used for jet engine turbines. See "High Temperature, High strength Nickel Base Alloys," July 1977. However, these alloys do not have sufficiently high temperature compression strength at temperatures of 1000.degree. C. or higher, despite the fact that they may be used in the atmosphere because of their high resistance to oxidation. Therefore, they cannot be used for isothermal forging dies which may be subjected to such high temperatures as 1000.degree. C. or more.
Nickel-molybdenum-aluminum alloys have also been proposed for use in gas turbine engines. Nickel-aluminum-molybdenum alloys produced by rapid solidification rate process (RSR) were disclosed by A. R. Cox, "Potential Uses of Rapidly Solidified Alloys in Gas Turbine Engines," Metals Technology, June 1980, pp. 238-243, and by R. J. Patterson II, "Rapid Solidification Rate Processing and Application to Turbine Engine Materials," Journal of Metals, September 1980, pp. 34-39. Also, a single crystal Ni-Mo-Al-Ta superalloy called MMT 143 was disclosed by D. D. Pearson et al., "Stress Coarsening of .gamma.' and Its Influence on Creep Properties of a Single Crystal Superalloy," Superalloys, 1980, pp. 513-520. All of these alloys were intended to be used for gas turbine engines.
It has never been intended to use nickel-aluminum-molybdenum alloys for isothermal forging dies which are to be operated repeatedly at temperatures of 1000.degree. C. or higher for a long period of time. Alloys for such isothermal forging dies must meet the requirements of high resistance to compression deformation and oxidation under severe forging conditions.